The recovery of crude oil present in deposits is generally carried out in several steps:
Production results first from the natural energy of the fluids and of the rock, which are decompressed. At the end of this depletion phase, the amount of oil recovered at the surface represents on average some 5 to 150 of the original oil in place. It is therefore necessary, in a second step, to employ techniques targeted at increasing the recovery yield while maintaining the pressure of the field.
The most frequently employed method consists in injecting water into the deposit via injection wells dedicated to this purpose. The term used is then secondary recovery. This second phase stops when the water content in the mixture produced by the output wells is too high. The gain here, in terms of degree of additional recovery, is about 10 to 20%.
The other techniques which can be used are combined under the name of enhanced oil recovery (EOR). Their aim is to recover between 10 and 35% of additional oil. The term “enhanced oil recovery” encompasses various thermal or nonthermal techniques, such as “electrical”, “miscible”, “steam” or “chemical” techniques for improved recovery of the oil remaining in place (cf. Oil and gas science and technology—IFP review, vol. 63, (2008), No. 1, pp. 9-19). The term “oil” should be understood to mean any type of oil, namely light oil and heavy oil, or even asphaltic oil.
The invention relates more specifically to the enhanced oil recovery by the chemical route involving at least the injection of water-soluble polymers in the form of a dilute solution.
The sweep efficiency by water injection is generally improved by adding water-soluble polymers. The expected and proven benefits of the use of polymers, through rendering the injected water “more viscous”, are the improvement in the flushing and the mobility control in the field, in order to recover the oil rapidly and efficiently. These polymers will increase the viscosity of the water.
It is known to those skilled in the art that synthetic water-soluble polymers, and in particular those which are acrylamide-based, are polymers that are highly advantageous for increasing the viscosity of aqueous solutions and are, in fact, used predominantly in enhanced oil recovery.
There are various processes of enhanced oil recovery. Mention may be made of the “surfactant polymer” or SP process or else the “alkaline surfactant polymer” or ASP process, in which processes an amount of surfactant is present in the aqueous, optionally alkaline, solution of water-soluble polymer which is injected into the well. These surfactants make it possible to reduce the surface tension between the oil of the underground formation and the injected solution. The alkaline agent, for its part, reacts with the compounds present in the crude oil, so as to form, in situ, supplementary surfactants which contribute to the emulsification of the oil.
Injection of the viscous solution is, in any event, continuous. At the end of a certain period of time, ranging from several weeks to several months, the polymer, the water and the oil emerge from the reservoir via the output well where the oil is then isolated. Large amounts of water are coproduced and it is also known practice to purify the water from the recovered oil and to recycle it for obvious economic and environmental reasons, but also sometimes for reasons of low water availability. Most commonly, the polymer solution injected is directly prepared on site, in proximity to the oil deposit in which it will be used. The recycled water, but also the seawater, river water, dam water or aquifer water directly available on the site will be directly used. It must be compatible with the polymer. This water must therefore be conditioned for the dissolution of the polymer, since the initial water most commonly contains oxygen, hydrogen sulfide and various other gases, iron and various other metals which are harmful to the stability of the polymer.
Indeed, in a manner known in the prior art, the chemical degradation of polymers is due mainly to the formation of free radicals which will react with the main chain of the polymer and create a drop in the molar mass. This results in a drop in viscosity of the polymer-based solution.
The presence of oxygen is the most harmful factor with regard to the degradation of the polymer. This reaction of polymer degradation by oxygen is amplified by the presence of metals such as iron, but also copper or nickel, or by the presence of hydrogen sulfide.
It is known practice, in the prior art, to use oxygen scavengers (sulfite, bisulfite, dithionite, hydrazine, etc.); the term “chemical deoxygenation” is then used. Mention may, moreover, be made of applications FR 2 945 542 and WO 2010/133258, in the name of the applicant, describing a composition based on anionic polymers or water-soluble amphoteric (co)polymers suitable for viscosifying the injection fluids for oil recovery. This composition further contains, before dilution with the injection fluid, at least 3 stabilizing agents chosen from the group comprising oxygen scavengers, precipitating agents, free-radical scavengers, complexing agents and sacrificial agents, said stabilizing agent being suitable for preventing the chemical degradation of the polymer once the polymer is introduced into the injection fluid. U.S. Pat. No. 4,795,575, for its part, describes an enhanced oil recovery process which provides for the injection of an alkaline aqueous solution substantially free of dissolved oxygen due to the incorporation of an oxygen scavenger, such as a bisulfite, present at between 100 and 500 ppm in the aqueous solution injected.
Nonchemical deoxygenation processes based on inert gases also exist. The gas displaces the oxygen present in the solution, either by bubbling, or by cycles of reequilibration of the pressure by this gas. These processes are often expensive and are not used in the context of the invention.
In concentrations of less than 50 ppb of dissolved oxygen, the degradation of the polymer is low and acceptable. In this case, a relatively low amount of oxygen scavenger, as envisioned by the applicant in its patent application FR2945542, is sufficient to neutralize the oxygen.
On the other hand, for concentrations above 50 ppb of oxygen, the degradation of the polymer is high and de facto decreases the effectiveness of the polymer-based solution, during the flushing of the oil well. It is therefore necessary to add larger amounts of oxygen scavengers.
It should also be emphasized that there are numerous sources of contamination of the water by dissolved oxygen (O2). There are sources termed “controllable” since they are either of intentional origin or of identified origin, and sources termed “uncontrollable” since they are not constant.
The “controllable” sources of O2 are, for example:                the initial presence of oxygen in the water (unintentional source);        the intentional oxygenation or purification (filtration, precipitation, etc.) of the water in order to remove the gases, metals and impurities contained in the water;        the oxygen contained in the aqueous solutions added into the circuit, for instance the alkaline solutions or the compositions based on surfactants, cosurfactants, solvents and/or cosolvents.        
The “uncontrollable” sources of oxygen are, for example:                oxygen leaks in the circuit;        oxygen leaks at the level of the polymer dissolving unit;        the oxygen present in the particles or grains of carbonate and/or polymer for example, added into the circuit, described as occluded oxygen.        
In an enhanced oil recovery process which involves the use of a viscous solution of water-soluble polymer(s), both the oxygen present in the water used to dissolve the various chemical products and also the redox potential of the deoxygenated water are regularly tested. However, since this type of test takes place on surface equipment, and the reaction for reduction of the oxygen by the oxygen scavengers is not instantaneous, it is not possible to know with certainty whether there is an oxygen leak, between the measurement of the oxygen level or of the redox potential, and the injection into the well. As a result, an excess of oxygen scavenger is generally added in order to guard against these possible leaks and these possible pollutions.
However, it is known to those skilled in the art that a large excess of the oxygen scavenger also leads to degradation of the polymer. Indeed, the reintroduction of oxygen into a system comprising a polymer in solution with an excess of oxygen scavenger is extremely harmful to the polymer. This is because an oxidation-reduction reaction is created between the O2 and the oxygen scavenger, giving radicals which then degrade the polymeric chain, this phenomenon being accentuated in the presence of Fe2+ ions. The degradation of the polymer can then be very rapid and the oil well flushing effectiveness is accordingly drastically affected. As it happens, the risks of oxygen reintroduction are high, given the various sources of oxygen contamination previously recalled.
It is therefore generally recommended to have a precise and controlled metering of oxygen scavenger, in order to have a slight stoichiometric excess, of about 10 to 20%, of this agent in the medium.
Moreover, in the prior art, the use of various stabilizing additives has been envisioned. Among them, the applicant has more specifically focused on the use of free-radical scavengers and sacrificial agents. Formulations containing water, a water-soluble polymer, an oxygen scavenger, a free-radical scavenger and a sacrificial agent have already been described in the prior art. Mention may in particular be made of the following documents:                U.S. Pat. No. 4,925,578 describes an aqueous composition with a pH greater than 10 containing an acrylamide polymer, an oxygen scavenger, a free-radical scavenger and an alcohol which can be used in enhanced oil recovery. The compounds are added through an alkaline brine, and the whole is mixed and used. The addition of oxygen scavenger is carried out at the same time as the other compounds. The use of this composition in the enhanced oil recovery field is described in U.S. Pat. No. 4,795,575;        U.S. Pat. No. 4,317,758 relates, for its part, to an aqueous solution containing an acrylamide polymer, an oxidizing agent/reducing agent pairing and a sulfur-containing stabilizing agent. It is also envisioned to use sodium hydrosulfite compounds as oxygen scavenger, thiourea as free-radical scavenger and mercaptoethanol, but no introduction sequence is presented as advantageous. Two different introduction sequences are nevertheless envisioned: one in which the polymer and the additives are added separately, and another in which all the additives are uniformly mixed with the polymer before the composition is used;        Sorbie in his work entitled “Polymer Improved Oil Recovery”, 2000, Blackie and Son Ltd (Glasgow and London), pages 90-104, describes a system for protecting biopolymers containing thiourea, isopropanol and sodium sulfite. A preferential sequence of introduction in an aqueous solution, in which the oxygen scavenger, then the free-radical scavengers and, finally, the polymer are successively added, is proposed;        Wellington, in SPE 9296, proposes stabilizing biopolymers by virtue of the addition of oxygen scavenger before the polymer is added, and of free-radical scavenger for protecting the polymer against any free radicals that might form during an accidental contamination with oxygen. Two particular sequences are envisioned: a first in which the polymer is added to a mixture of water, oxygen scavenger and free-radical scavenger, and a second sequence in which the polymer is added to an aqueous solution containing an excess of an oxygen scavenger, and then this solution is diluted in an aqueous solution containing a free-radical scavenger and a sacrificial agent. Wellington also describes the synergistic effect linked to the combination of thiourea (free-radical scavenger) and PA (sacrificial agent);        Shupe in SPE9299 describes how to stabilize polyacrylamides by testing the effects of various additives, without, however, testing complex combinations and without giving an introduction sequence.        
Nevertheless, the applicant has demonstrated that the various processes for preparing an aqueous composition of water-soluble polymer(s) that are described in the abovementioned documents do not enable an optimal protection of the water-soluble polymers used in oil fields. There remains therefore a need for a solution which makes it possible to obtain maximum protection of the polymers, knowing that the number of sequences combining the addition of polymer and of stabilizing additives envisioned in the prior art can amount to thousands.